Directory UMM :Data Elmu:jurnal:J-a:Journal of Asian Earth Science:Vol19.Issue1-2.Feb2001:

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towards the MCT changes either to plane strain or in some cases to constrictional strain. This change in strain is well recorded in the microstructures. The zone dominated by ¯attening strain is expressed as bedding parallel mylonites. The grain reduction in this zone has occurred by dynamic recrystallization and quartz porphyroclasts were ¯attened parallel to the mylonite zone. The maximum ®nite strain ratio observed in this zone is 2.2:1.8:1. The zone, where the ¯attening strain changes either to plane strain or constrictional strain, record an increase in ®nite strain ratio up to 3.8:1.9:1. This zone represents deformation fabrics like S±C microstructures simultaneously developed during mylonitization in an intense ductile shear zone. The above zone is either near the MCT or adjacent to crystalline klippen occupying the core of the synforms in the footwall of the MCT. The microstructural evolution and the ®nite strain suggest that the MCT has evolved as the result of superposition of southward directed simple shear over the ¯attening strain. The simple shear has played an active role in the rapid translation which followed the mylonitization at deeper levels.q2001 Elsevier Science Ltd. All rights reserved.

1. Introduction

The collision of the Indian and Asian plates during the Palaeogene period produced several thrust planes. The

differential movements along these tectonic planes

produced composite thrust sheets. The crustal shortening, thus produced, was concentrated on a few major thrust zones located south of the Indus Tsangpo suture zone. One of the most important of these is the Main Central Thrust (MCT). Along this thrust, medium to high grade

metamorphic rocks (Higher Himalayan Crystallines:

HHC) were thrusted over the low grade meta-sedimentary rocks of the Lesser Himalaya (Heim and Gansser, 1939; Gansser, 1964; Le Fort, 1975; Thakur, 1987). On the basis of the presence of klippen of the HHC far south over the Lesser Himalayan rocks, the MCT zone is thought to have a cumulative displacement of at least 140±210 km (e.g. Schelling and Arita, 1991; Schelling, 1992). Studies regard-ing the crustal shortenregard-ing and deformation fabrics produced during this displacement have been limited in number. The present knowledge about the MCT and other related thrusts in general, is either restricted to deformation fabrics (Bouchez and Pecher, 1981; Brunel, 1986) or strain pattern

(Roy, 1980; Saklani and Nainwal, 1989; Jain and Anand, 1988; Singh, 1991). The microstructural changes with strain pattern are required to better understand the internal defor-mation associated with the MCT. The western part of the Garhwal Himalaya between Tons and Bhagirathi valleys were selected for the above studies, since strain markers are known here across the MCT (Singh, 1991). The footwall of the MCT has klippen of the thrust sheet rooted within the crystallines of the HHC. An integrated study of strain pattern, mylonitization and microstructures across the MCT zone and also its footwall hold the key to understand the nature of deformation associated with the MCT in this part of the Garhwal Himalaya.

2. Geological setting

The geology of Garhwal Himalaya is dominated by a series of northward dipping and S-verging thrusts. The Main Central Thrust (MCT), one of the prominent thrusts, has long been recognized as a major intracontinental ductile shear zone with an associated inverted metamorphic zone (Heim and Gansser, 1939; Valdiya, 1980 etc.). The MCT separates two geologically distinct zones i.e., the Lesser Himalaya (south of the MCT) and the Higher Himalayan Crystallines (north of the MCT) (Fig. 1). The rocks of the

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Lesser Himalaya are largely Precambrian sedimentary units with a few outcrops of Cambrian and Tertiary strata. The Higher Himalayan Crystallines (HHC) rocks (1800±2300 Ma old: Bhanot et al., 1980; Singh et al., 1986) occur as a large, 15±20 km thick thrust sheet and are composed of two major lithotectonic zones. The underlying inverted meta-morphic zone is dominated by feldspar rich augen gneisses, granodiorite, metasediments, amphibolites and is known as Munsiari Thrust Sheet. The tectonically overlying prograde sequence constitutes the high grade metamorphic zone mainly the kyanite±sillimanite±garnet gneiss, psammitic gneiss, granulite, magmatite and is known as the Vaikrita Thrust Sheet. There has been controversy over the exact location of the MCT. Heim and Gansser (1939) ®rst de®ned it as the thrust, which places the Crystalline Nappes of the Higher Himalaya over the sediments of the Lesser Hima-laya. Valdiya (1980) maps the Vaikrita Thrust (VT) as the MCT in the Kumaun Himalaya. This is in agreement with the tectonic position of the MCT in the Nepal Himalaya (Le Fort, 1975; Pecher, 1977). Sinha Roy (1982) placed the MCT at the base of the inverted metamorphic sequence in the Eastern Himalaya. In Zanskar Himalaya, the MCT is located at the base of undifferentiated crystalline rocks, which wrap the Lesser Himalayan rocks in the Kishtwar Window (Thakur, 1998; Searle and Rex, 1989). The MCT, in Western Garhwal, is also recognized as thick one (the MCT zone: up to 12 km) in which the metamorphic isograds were inverted (Bouchez and Pecher, 1981; Mohan et al., 1989; Metcalfe, 1993; Searle et al., 1993). This zone,

however, is part of the earlier known Central Crystallines (Gansser, 1964; Valdiya, 1980). The lower boundary of the MCT zone (the MCT for present description) corresponds to the Munsiari Thrust (MT) and the upper boundary to the Vaikrita Thrust (VT). The HHC rocks are bounded to the North by the South Tibetan Detachment (STD: Searle, 1986; Valdiya, 1989).

The MCT (MT: Valdiya, 1980) in the studied area has translated the inverted metamorphic rocks of the HHC southward over the quartzite of the Berinag Thrust Sheet (BTS) of the Lesser Himalaya (LS). It dips at about 40₈to 558NE or NNE and the rocks show mylonitic fabric both along the hangingwall and the footwall. The inverted meta-morphic rocks with thickness of several km form the MCT zone. The lower boundary of the MCT zone (i.e., the MCT) is observed around Wazri (Fig. 2), Mori (Fig. 3) and Sainj (Fig. 4) in the Western Garhwal Himalaya. The rocks of the MCT zone exposed at the higher structural level are within the amphibolite facies, whereas the base of this structural pile are in the lower greenschist facies. This observation is in agreement with the already known metamorphic assem-blages in the Bhagirathi valley (Metcalfe, 1993; Searle, 1986) and the other parts of the Himalaya (Hubbard, 1989; Le Fort, 1975; Mohan et al., 1989; Searle and Rex, 1989). A variety of shear zones ranging from mm to outcrop scale are generally parallel or at acute angle to the schist-osity in the MCT zone. These zones are associated mainly with the change in the lithology in these rocks. The geometry of the feldspar porphyroclasts in the gneiss

K. Singh, V.C. Thakur / Journal of Asian Earth Sciences 19 (2001) 17±29 18

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show abundant evidence of top to south sense of shear (Metcalfe, 1993) as a result of ductile synmetamorphic strain. The unit structurally below the MCT to the South, comprise the quartzite dominated Berinag Thrust Sheet (Valdiya, 1980) of the Lesser Himalaya. The quartzite is massive, coarse grained, at places sericitized and generally white or pale white in colour. This sequence is thrust over the limestone rich Deoban Formation (Valdiya, 1980) along the Berinag Thrust (BT). Over the quartzite of the Berinag Thrust Sheet lies a number of klippen of the Higher Hima-layan Crystallines. These klippen are the remnant of the Chail and or Jutogh Thrust Sheets derived from the HHC. Two klippen of the Chail Thrust Sheet comprise of chlorite and muscovite schists, described here as Kanaera Klippe (Fig. 2) and Chatra Klippe (Fig. 3) along the Yamuna and Tons valleys respectively. Another klippe of the Jutogh Thrust Sheet composed of biotite schists, quartz±muscovite schist and coarse grained gneiss is the Purola Klippe.

The structural analyses at mesoscopic and microscopic scale in the MCT zone and the part of the Lesser Himalayan sequence (Berinag Thrust Sheet) forming the footwall indi-cate three phases of deformation. The earliest one (D1 defor-mation) has developed penetrative fabric (i.e., widespread

foliation and stretching lineation) observed in both the Beri-nag Thrust Sheet and the MCT zone. The main planer fabric (i.e., the foliation in the Lesser Himalaya and the schistosity in the MCT zone) is dipping NE at moderate to high angles. It is axial planer to the tight to isoclinal folds, plunging either NW or SE at moderate angles. The hinges of these folds (F1) are thickened and appear to be ¯attened. The foliation plane contains a NE plunging stretching lineation (L1) expressed by the highly stretched feldspar porphyro-clasts in the crystallines of the MCT zones and elongated quartz clasts in the Berinag Thrust Sheet. The relationship of stretching lineation (L1) and the foliation (F1) on the equal area net are represented for the Yamuna (Fig. 2), the Tons (Fig. 3) and the Bhagirathi (Fig. 4) valleys. The stretching lineation is also more strongly developed adjacent to the MCT and below the klippen. The linear fabric represents the `X'-axis of the strain ellipsoid in the direction of tectonic transport and is transverse to the trend of the MCT. The stretching lineation adjacent to the MCT, sometimes, becomes randomly oriented and almost becomes parallel to the strike as observed near Sainj along the Bhagirathi valley. The axes of the F1 folds are nearly normal to the L1direction. The quartz clasts in the quartzite progressively

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Fig. 3. Geological and structural map around the MCT in the Tons valley.

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become ¯attened when approaching the MCT as is the mylonitic fabric. The mylonitic banding in the footwall, as well as the MCT zone, is parallel to the MCT. Additionally the mylonitic fabric adjacent to the MCT changes to S±C microstructures (Berthe et al., 1979; Blenkinsop and Treloar, 1995). This change in the fabric is also observed adjacent to the crystalline klippen observed over the quart-zite, south of the MCT. The inverted metamorphic rocks of the MCT zone have several discrete zones where the S±C fabric is observed. Such zones are abundant in the porphyr-oclastic gneiss and mica schist. The geometry of the feldspar porphyroclasts and the S±C fabrics indicate abundant evidence of a top of the south sense of shear. A number of fold (F2) with large amplitudes were observed in the Berinag Thrust Sheet on the footwall side of the MCT. The synform within the quartzite (BTS) accounts for the klippen of both the Jutogh and Chail Thrust Sheets, and the antiforms as the windows of the underthrust Deoban Formation. The Kanera Klippe occurring over the quartzite in the Yamuna valley (Fig. 2) is occupying the synformal core plunging SE at a moderate angle. Similarly the Chatra Klippe in the Tons valley (Fig. 3) is also lying over the quartzite in a synformal core plunging SE at a moderate angle. The regional schis-tocity (S1) is folded by these folds (F2). Their limbs have developed crenulation foliation (S2). These F2 folds are interpreted as the result of successive and progressive south-ward translation of the HHC along the MCT. The D3 defor-mation is non-penetrative and observed as large very open folds and also as kink folds.

3. Finite strain analysis

3.1. Choice of strain markers and method

The ®nite strain was determined from quartz clasts of the quartzite of the Berinag Thrust Sheet occurring along the footwall zone and from the augen of the augen gneiss of

the MCT zone. The two dimensional strain was determined from each of the three mutually perpendicular thin sections assuming the foliation plane as theXYplane and stretching lineation as the `X'-direction for each hand specimen collected across the MCT (Siddans, 1972; Wood, 1974; Tullis, 1976). The presence of quartz subgrains along the margins of the quartz clasts makes it dif®cult to measure accurately their axial ratios. Therefore the `Center to Center' technique (Fry, 1979; Ramsay and Huber, 1983) was used to determine the ®nite strain from the quartz clasts of the quartzite. About 75±110 grain centres were recorded from enlarged photomicrographs for the strain determina-tion. The quartz clasts used in this analysis were large detri-tal grains surrounded by ®ne grains (30±40mm) of quartz

matrix. The clast sizes are at least an order of magnitude larger than the matrix grains. Most of the measurements were made onXZandYZsections, although measurements on someXYsections were also made to check the accuracy of the computation. By equatingZto unity two-dimensional strain ratios were integrated into the three dimensional strain (Ramsay and Huber, 1983). The axial ratios and orientations of the porphyroclasts were measured accu-rately, therefore Rf/B (Dunnet, 1969) technique was applied to determine the strain ratios in the MCT zone. The stretching lineation served as the arbitrary reference line for taking the orientation of the porphyroclasts. The shape of the strain ellipsoid was determined from the plot-ting of the ®nite strain ratios on Flinn and Hsu graphs (Hossack, 1968; Flinn, 1978).

3.2. Finite strain variation across the footwall of the MCT

The ®nite strain is determined from `26' oriented samples of the quartzite from the BTS occurring across the footwall of the MCT. These samples are distributed along three valleys (Fig. 5). The quartzite is about 5 km thick in the Yamuna valley and 6 km thick each in the Tons and Bhagir-athi valleys. The ®nite strain ellipsoids along the Tons,

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Yamuna and Bhagirathi valleys were plotted over the geolo-gical map (Fig. 6).

The ®nite strain is determined from `10' quartzite samples distributed along the Yamuna valley. The ®nite strain ratios near the base of the quartzite sequence is 1.9:1.7:1. It increases to 3.8:1.9:1 towards the Kanera Klippe and to 2.3:1.8:1 towards the MCT (Figs. 6 and 7B). The corresponding `K'-value adjacent to the Kanera Klippe is 1.08 (apparent constriction ®eld) in a progres-sively increasing trend from 0.21 (near the base) to 0.57 (apparent ¯attening ®eld) toward the MCT along the Yamuna valley (Fig. 8B). In this section the strain intensity (Es) values (e.g. Hossack, 1968; Flinn, 1978) corresponding to quartzite samples adjacent to Kanera Klippe is 2.02 in progressively increasing values from 0.6 to 0.93 toward the MCT. The ®nite strain data is also plotted on the Hsu Plot (Fig. 8b) which corresponds well to the Flinn plot (Fig. 8B). A near similar trend in the ®nite strain ratios is also observed in the quartzite sequence along the Tons valley. The ®nite strain is determined from `9' samples distributed from the base to the MCT along above valley. The ®nite strain near the base of the quartzite sequence is 1.9:1.7:1. It increases to 2.5:1.7:1 towards Chatra Klippe and to 2.3:1.7:1 towards the MCT (Figs. 6 and 7A) along the Tons valley. The corresponding `K' values adjacent to Chatra Klippe is 0.55 in a progressively increasing from 0.2 to 0.6 towards the MCT (Fig. 8a). TheEsvalue adjacent to Chatra Klippe is 1.0 in a progressively decreasing value from 0.6 to 0.48 toward the MCT. The ®nite strain data were also plotted on Hsu plot (Fig. 8a) which correspond well to Flinn plot (Fig. 8A).

The ®nite strain trend observed in the `8' quartzite samples along the Bhagirathi valley indicates a consistent and progressive increase from 1.8:1.6:1 to 2.5:2.0:1 towards

the MCT (Figs. 6 and 7C). The corresponding `K' values progressively increase from 0.25 to 0.48 in the ¯attening ®eld (Fig. 8C) except for sample UM 20 collected adjacent to the MCT which gives a lower values, i.e. 0.14. The Es values also represent an increase from 0.64 to 0.9 towards the MCT. The spatial distribution of variation in the Es values are represented in Fig. 9.

3.3. Finite strain variation across the MCT zone

The ®nite strain (Rf/Btechnique) is determined from the stretched porphyroclasts (more than 30 readings) at six localities along the Yamuna valley and four in the Tons valley from the prophyroclastic gneiss of the MCT zone. It varies from 2.3:1.9:1 to 2.0:1.9:1 adjacent to the MCT and decreases to 1.9:1.7:1 structurally upsection in the Yamuna valley. It is 2.2:1.9:1 adjacent to the MCT and decreases to 2:1 1:7:1 structurally upsection in the Tons valley. The above ®nite strain is in agreement with the already known ®nite strain in the Tons Valley. The two dimensional ®nite strain determined from the augen decreases from 2.1:1 (X:Z) to 1.6:1 upsection above the MCT (Jain and Anand, 1988). The above results indicate an increase in the ®nite strain towards the MCT.

3.4. Microstructural evolution based on strain variation across the footwall of the MCT

The microstructural variation with progressively increas-ingEsvalues toward the MCT were studied along all three valleys (i.e. the Tons, the Yamuna and the Bhagirathi valleys). The shear fabric study in relation to the foliation (S1), the stretching lineation (L1) and the S±C fabrics indi-cate a progressive deformation history related to MCT as described below.

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The microstructural variation in the quartzite was described in accordance withEsvalues along the footwall of the MCT. The microstructural changes in the quartzite with different strain intensities (Es) in a progressive defor-mation suggest that with overall strain intensity (Es<0.7) observed in the basal part of the quartzite sequence, the quartz clasts are relatively less deformed and show undulose extinction (Fig. 10A). The ¯attened clasts indicate weak foliation planes. Recrystallized quartz grains are less than 10% of the volume and restricted to the clast margins. Some of the clasts show diffusive boundaries because of develop-ment of quartz subgrains in the marginal zone.

With increasing strain (Es<0.9) the volume percentage of the recrystallized quartz subgrains increases and varies from 30 to 45%. The size of the recrystallized grains appears to be fairly constant at approximately 20±30mm. The

mylo-nitic foliation is de®ned by the aligned, and relatively more ¯attened clasts and by the recrystallized quartz grains (Fig. 10B). The quartz subgrains are also developed along the undulose extinction planes in some of the highly stretched, deformed quartz clasts. The ®nite strain up to this level is still in the ¯attening ®eld.

The recrystallization of the clasts into subgrains

contri-buting to the matrix is 50±65% in the highest recorded strain (Es<1.1) occurring adjacent to the MCT. The ¯at-tening strain in this zone change to, or have a tendency toward plane strain. The quartzite of the Berinag Thrust Sheet with strain intensity (Es<1.2) was observed near the contact zone with the klippen of the Chail Thrust Sheet. The ®nite strain symmetry here also changes to constriction. The microstructures in the zone with

Es<1.1 or Es<1.2 mark the appearance of S±C fabrics and are therefore described together. The thickness of this zone is about 200 m and followed above by a highly deformed and nearly totally recrystallized zone with well developed S±C fabrics (cf. Berthe et al., 1979; White et al., 1980; Lister and Snoke, 1984; Blenkinsop and Treloar, 1995) adjacent to the MCT. The quartz clasts in the zone are highly elongated with distinct undulose extinction zones bounded by the microfracture planes (Fig. 10C). The devel-opment of subgrains is intense, sometimes leading to nack-ing within the clasts (Fig. 10D). The majority of the quartz clasts in the zone are oriented parallel to the `S' planes and are asymmetrically wrapped by the mylonitic foliation (Fig. 11A). The S±C fabrics, similar to `C' shear band (Passchier and Trouw, 1996), is developed within some of the quartz

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clasts (Fig. 11C). The excessive recrystallization along the microfracture planes change the clasts into sigmoidal shaped subclasts. The shape of such a divided quartz clast give a top to south sense of shear movement. The quartz

clasts in the zone closer to the MCT no longer preserved their outer boundaries because of the development of a higher percentage of quartz subgrains. Still the quartz clasts that escaped recrystallization present a higher axial ratio

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indirectly indicating a further increase in the ®nite strain ratio towards the MCT. The `S' surfaces in the S±C fabrics are parallel to the S1plane outside this zone and `C' surfaces parallel to the shear plane (Fig. 11B and C) similar to that of Berthe et al. (1979) model. Generally the intersection between `S' and `C' is at a moderate to high angle. The `S' surfaces within the shear zone are identical in

appear-ance and continuous with outside S1foliation. This indicates that the slip occur only along the `C' plane and not along the `S' plane.

Common microstructures of S±C fabric are mica ®sh observed adjacent to both the MCT as well as the zone very near the klippen. The mica ®sh are aligned parallel to the `S' planes while their thin, long and narrow trails are

Fig. 9. Contouring of the ®nite strain intensity (Es) along the MCT superposed over a geological map.

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merging into the `C' planes (Fig. 11B). The 001 plane of the mineral cleavage within both symmetrical (Fig. 11B) and asymmetrical mica ®sh (Fig. 11D) are curved as a result of slip along these planes. In some mica ®sh, the 001 planes are acutely transected by another shear plane (C±C: Fig. 11E) observed along the outer extremity. Such shear planes displace the cleavage and die out within the mica ®sh. The alignment of these shear planes are almost parallel to the C planes (Lister and Snoke, 1984). The recrystallized mica is observed within some of the mica ®sh. Such newly crystallized mica grains are aligned at an acute angle along the plane of maximum extension (Fig. 11F). The geometry

and orientation of new mica indicate that growth took place during the late strain-softening period. The sense of shear deduced from such grains is the same as that recorded by the asymmetrical mica ®sh geometry. The presence of the microshear planes and growth of new mica within the mica ®sh indicate a late stage of shearing. Such evolution is supported by other elements in the mylonitic fabrics, such as the sense of rotation of quartz clasts and the syn-shearing folds. The microstructural observations in the mica ®sh of the S±C fabrics are indicative of typical non-coaxial deformation.

The correlation of strain intensities with the development

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(the ®nite strain is determined from such porphyroclasts) are generally observed in the protomylonite. The mica schist adjacent to the MCT of the Yamuna and Tons valleys corre-spond to high strain zone and consist of thin elongated plates of quartz within ®ne grained muscovitic aggregates (ªshim-mer aggregatesº). The S±C fabrics are also well developed in the mica schist. The overlying porphyroclastic gneiss with ultramylonitic fabrics sometimes, exhibit S±C fabrics. The porphyroclasts of the feldspar consist of tails of pres-sure shadows at their end mainly in the protomylonite. The polygonized quartz grains are observed within the shadow zones. The large sized porphyroclasts show both the fracture as well as stretching. The recrystallization within the porphyroclasts is generally con®ned along the fracture planes. The feldspar porphyroclasts sometimes show undu-latory extinction. Therefore these clasts appear to have deformed by the combined effect of plasticity and fractur-ing. The aspect ratios of some of the feldspar porphyroclasts (in cm.: not used in strain calculation) is up to 3:1 and 8:1 in some of the extremely stretched pull-apart porphyroclasts.

4. Strain controlled microstructural pro®le across the MCT and the inverted metamorphism

The zonal distribution of the microstructures re¯ects the amount and type of ®nite strain at a given level in the quart-zite occurring along the footwall of the MCT. From the base of the quartzite sequence, the development of mylonitic characters are essentially governed by the strain pattern as indicated by the contouring ofEsvalues (Fig. 9). The zone between strain intensities (Es) 0.7 and 0.9 indicates that the strain is of a ¯attening type and there is a gradual increase in the ®nite strain from 1.9:1.7:1 to 2.2:1.8:1 with a nearly constant orientation towards the MCT. This zone represents nearly 40% recrystallization along the quartz clasts and the recrystallized quartz subgrains along these quartz clasts increases with the increasing strain. This increase in quartz subgrains aligns preferentially with higher strain and thus progressively develops the mylonitic foliation (S1) in the quartzite. The relationship between the microstructures and strain pattern in the quartzite indicates a direct bearing

ellipsoids which in turn is re¯ected in a stronger develop-ment of stretching adjacent to the MCT. The deformational changes along the quartzite below the crystalline klippen along the footwall of the MCT also indicate an increase in the strain and a change in strain symmetry from ¯attening to constriction (e.g. Chatra and Kanera Klippen). The S±C fabrics is also observed in this zone. It can be safely assumed that similar deformation conditions must have prevailed during translation of the Chail (also the Jutogh) thrust sheet from the HHC. Therefore the zone with a higher strain and a strong component of simple shear has triggered the translation of crystallines from the MCT zone to now lie over the footwall in the eroded form of klippen. This also supports the contention that the stretching lineation repre-sents the tectonic transport direction. The MCT zone has numerous zones rich in S±C fabrics mainly in the ultramy-lonitic fabrics. Such zones, in particular and the MCT zone as a whole can be interpreted as evolved from the super-position of simple shear over the ¯attening strain. Similar change in the deformation pattern were cited in the N Apen-nine shear zone (Klig®eld et al., 1981), Caledonian Skerrols Thrust (Saha, 1989).

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metamorphic zonation. This is explained by a higher rate of uplift and rapid erosion. A close relationship between the strain accumulation and metamorphic inversion is proposed along the MCT. The study of rocks indicates a mylonitized zone distributed all along the MCT zone as well as in a part of the footwall zone. The myylonitized zone with S±C fabrics indicate an increase in the strain and a stronger development of stretching lineation adjacent to the MCT, which is explained here as superposition of simple shear over the ¯attening strain. This simple shear has played an active role in the rapid translation of crystallines along the MCT. It also goes well with a rapid exhumation, which followed the mylonitization at dipper level.

5. Conclusions

The MCT and associated thrusts evolved in a progressive non-coaxial type deformation. The correlation of ®nite strain and microstructures indicate that the ¯attening strain progressively increased towards the MCT so does the mylo-nitic signatures. The ¯attening strain adjacent to the MCT and the crystalline klippen changes to either plane strain or constrictional strain and this zone is dominated by S±C microstructures. Such a strain pattern and microstructural evolution is explained here by superposition of simple shear over the ¯attening strain. The simple shear strain adjacent to the boundaries of the MCT zone has resulted from a rapid translation.

Acknowledgements

KS is grateful to Dr. M.I. Bhat for his constructive comments and detailed review of the paper. He is also thankful to the Director, Wadia Institute of Himalayan Geology, Dehradun for providing various facilities.

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